Image registration is the process of aligning two or more images so they accurately coincide with one another. It is routinely required for purposes of aerial identification and targeting, medical diagnosis and treatment, and geophysical studies. The registration of images of the same object obtained using different techniques (e.g., radar, optical, laser illuminated, magnetic resonance imaging (MRI), x-ray, etc.) is generally very difficult because it is not generally tractable. This multimodal registration is even more difficult if the initial uncertainty window in registration is large enough that a direct correlation coefficient calculation encounters numerous false maxima over said window. Numerous R^2 maxima in the uncertainty window can be handled by some methods, but if the highest R^2 maxima does not correspond to the correct image registration, then resulting images are likely to be misaligned. In situations where there is human oversight, this can be rectified, but in time constrained circumstances (i.e., not an automatic process). Further, image misregistration leads directly to munition mistargeting with destructive consequences. Synthetic aperture radar (SAR) imagery is notable for its wide dynamic range (typically 16 bits) and strong coherence effects (esp. speckling) while optical imagery (as photographed in natural light) has lower dynamic range (typically 8 bits) but more channels (RGB) and an absence of coherent effects. Additional differences are scattering and reflectivity responses of the physical objects in the imaged scenes at different wavelengths (λ=3 cm/0.00005 cm at radar/optical) and the distinctly different location of shadows (they are the same only if the optical images are acquired with the imaging platform directly between the illumination source (sun) and the scene). This difference and variety in response characteristics can manifest itself as a mapping or correspondence between the SAR and optical signals that depends strongly on position within an imaged scene.
One class of approaches to SAR/optical registration identifies distinct features or control points in each image, matches or corresponds these features one to another, and then aligns the images based on these matched features. Feature based imaging techniques are difficult because of multiplicative SAR speckle and the general lack of a uniform correspondence between optical and radar features. Exactly how these features are chosen is typically algorithm-dependent and said features sometimes go by the name of bandlets, beamlets, wavelets, and tip points. These features are chosen manually or automatically and are supposed to be salient and distinctive objects shared by both images (target and reference). Their density and location is entirely image dependent.
Another class of approaches utilizes edges or more generally contours or contourlets, curvelets, ridgelets as the matching features. Because of coherence effects (corner and edge reflections), some contours in SAR can be over prominent (bright lines or streaks) relative to optical imagery. Strong speckling effects leads to additional contours without corresponding optical contours. Overall, this results in extraneous SAR edges which must be pruned prior to feature matching. Typical of these approaches is the suppression of SAR speckle using adaptive spatial moving averages which in the case of a single image always leads to a reduction in image resolution.
For registration of imagery not displaced by much, the above algorithms may be suitable but what is lacking is a robust method for registering images with large uncertainty windows.